There are conventionally known running control devices that grasp traffic situation ahead by vehicle-to-vehicle information communication and control various devices present in a vehicle based on this information. Some of such running control devices are based on automobile-to-automobile communication. Such a running control device in a subject vehicle includes: a receiving means that receives information from another vehicle present ahead; a transmitting means that transmits the result, which is obtained by processing information on vehicles running ahead and the subject vehicle, to other vehicles present behind; and a running information processing means connected with various controllers present in the subject vehicle. The running control device is so constructed that it assists the safe running of the subject vehicle by collecting information on nearby vehicles and controlling the various controllers in the subject vehicle.
(Refer to Patent Document 1, for example.)
Some of such running control devices include in a subject vehicle: a front light transmitting/receiving unit that transmits or receives laser light signals to and from the area ahead of the subject vehicle; a rear light transmitting/receiving unit that transmits or receives laser light signals to and from the area behind the subject vehicle; and a signal relaying means that relays signals between the front light transmitting/receiving unit and the rear light transmitting/receiving unit. In this configuration, laser light signals including signals received by the front light transmitting/receiving unit are transmitted from the rear light transmitting/receiving unit; and laser light signals including signals received by the rear light transmitting/receiving unit are transmitted from the front light transmitting/receiving unit. Thus, the running control device makes it possible to pass a signal from one vehicle to another in rotation. (Refer to Patent Document 2, for example.)                Patent Document 1: JP-H05-266399 A        Patent Document 2: JP-H09-051309 A        
However, the above-mentioned running control devices involve a problem. When there are multiple vehicles ahead, laser light is prone to be intercepted by those vehicles; therefore, it is difficult to appropriately carry out communication by light between vehicles. Also, when light from an oncoming vehicle is received as noise, it is difficult to appropriately carry out communication by laser light.